Why do apples turn brown?! Let’s experiment and then learn more (see BUT WHY!?! at the end!)
Apples and Lemon Juice Experiment
- Lemon juice
- Small bowl (able to fit half the apple)
- Clock or timer
- Paper plates
- With a grownup’s help, cut the apple in half from top to bottom.
- Fill the bowl with enough lemon juice to cover the bottom.
- Place one half of the apple, cut-side down, in the bowl of lemon juice. Leave the apple in the lemon juice while you do the next step.
- Label your one plate “Lemon Juice” and the other “Control”
- Place the apple that is soaking in the bowl of lemon on the “lemon juice” plate. Place the other apple half on the “control” plate.
The lemon juice is the variable, the one thing that is changed, in this experiment.
- Observe the color of both apple halves at the start of the experiment.
- Set a timer for 15 minutes. Observe the apples when the timer goes off.
- Repeat this step [set timer for 15 minutes and observe when the timer goes off] for three more cycles.
Each time the timer goes off, look at the apples but do not touch them.
Do you see any changes in color or texture? What do you observe?
- Look at the apples again periodically throughout the day. What do you discover?
When you cut an apple, or another fruit that has a skin or covering, you expose the insides to the air. Within the air, there is lots of oxygen (thank goodness for us, bad for the fruit).
The inside, or the meat of the fruit, contains an enzyme called polyphenol oxidase (abbreviated: PPO). When this enzyme comes into contact with the oxygen in the air, it sets off a chemical reaction that causes the fruit to turn brown. This can happen by cutting into it or could happen if the fruit gets dropped or bruised.
Does that mean the fruit is no longer good to eat?? Yuck, who wants to eat brown fruit, right? But the answer is “Of course it is still good! It is just a different color.”
But what if we want our fruit to stay the original color? Can we do that? Actually, yes, using a little bit of science!
We know the fruit turns brown when it touches the air, but what if we soak it in water or cover it with a liquid? That would stop the oxygen from reaching it. But… who wants to eat soggy fruit?
Let’s think a little more…
The reaction between the enzyme and the oxygen actually needs a certain environment to happen. This reaction is called enzymatic browning and it can only happen within a specific pH window. The enzymatic browning can’t happen if the pH is low.
If we coat the fruit with a liquid that has a low pH, we can slow down the process! If we coat the apple with a juice with a low pH, it will preserve the original color longer! The lemon juice that you used for the experiment has a pH of 2.
Do you think this will work with any type of juice? Try it and see!
If you are a member of the Discovery Club, you can use your cabbage juice pH indicator to find other juices or things you can coat your apple with that have a low pH.
To learn more about acids, bases, and pH, we invite you to join the WMFK Discovery Club where the month of February is all about pHun science! (Get it? pHun=fun!)
See you at the Hub!